1. Field of the Invention
The present invention pertains to the field of high-power semiconductor lasers, laser arrays, and amplifiers.
2. Description of the Prior Art
Broad area semiconductor lasers contain a laterally-extended gain region within the resonant optical cavity of the laser. Because broad area lasers have larger gain volumes than conventional narrow waveguide, single spatial mode semiconductor lasers, these lasers are capable of achieving higher output power. However, as a rule, if the lateral width of the active region in a broad area laser is greater than about 5 xcexcm, the optical resonant cavity of the laser is capable of supporting multiple spatial modes. The lateral boundary conditions for the electromagnetic wave, which play the dominant role in selecting a single spatial mode in narrow waveguide lasers, do not play a significant role in mode selection for broad area lasers. Additionally, when the lateral width of the active region is greater than about 50 xcexcm, the relative gain thresholds of different spatial modes are very close to one another. As a result, a number of lateral spatial modes (typically in the 10s) are simultaneously excited in broad area lasers with a lateral dimension of greater than about 50 xcexcm, even at moderate pumping levels. The number of excited lateral spatial modes increases as the pumping level increases, so that the output optical power in an individual lateral mode tends to saturate or doesn""t increase at all.
In broad area semiconductor lasers, the lateral spatial distribution of laser energy is further complicated by an optical nonlinearity in the semiconductor laser gain region, due to the dependence of the refractive index on the free-charge carrier density. This nonlinearity is responsible for the self-deformation of the lateral mode field distribution, so that the lateral extent of the active region can have areas of higher and lower light intensity, often called filaments. The physical reason for the appearance of filaments can be understood as follows: Suppose light intensity increases slightly in a small part of the active region, for example due to an intensity fluctuation or a slight inhomogeneity in the epitaxial structure of the semiconductor laser. In this region, the local carrier density will be decreased below that in nearby ambient areas, due to the higher rate of stimulated recombination. The decreased carrier density in this local area causes a corresponding increase in the refractive index. As a result of this local increase in the refractive index, light rays will be deflected into the local area from adjacent parts of the active region, further increasing the optical intensity. This reinforces the original effect and an instability in the homogeneous lateral distribution of the optical intensity appears, leading to the creation of filaments.
An important feature of filamentation in broad area semiconductor lasers is the periodicity of the filaments, with an observed spacing between nearest filaments typically in the range of 10-20 xcexcm in the lateral direction. This observed periodicity can be described in a series of spatial Fourier harmonics. In these terms, one can consider the appearance of filamentation in a broad area laser as the appearance of perturbed waves, where each wave represents a separate Fourier harmonic. The actual spatial period of the filamentation is determined by the phase matching conditions between the main laser mode, which propagates parallel to the optical cavity axis, and the perturbed waves, which propagate at some angle with respect to the axis. Thus, there are two physical features that are responsible for the appearance and behavior of intensity filamentation in broad area semiconductor lasers. The first is the optical nonlinearity of the semiconductor gain media due to the dependence of the refractive index on carrier density, and the second is the phase matching condition for waves which are affected by the nonlinear interaction.
The phase matching requirement is a common condition in almost all optical nonlinear processes such as, for example, the generation of optical mixing frequencies, stimulated Brillouin scattering, and stimulated Raman scattering. These processes can take place in many different media in which a nonlinear effect can be achieved, not only semiconductor gain media, but also transparent media such as dielectric crystals, gases, etc.
As indicated above, the existence of many excited lateral modes in broad area semiconductor lasers and the tendency for intensity filamentation lead to the generation of a very complex and nonuniform lateral intensity distribution. This is an undesirable property of conventional broad area semiconductor lasers mainly due to two effects. The first is the decrease in the reliability, or lifetime, of the operating laser. In the localized area of the laser cavity mirror (often a facet of the semiconductor laser diode structure) which corresponds to the center of a filament, the light intensity can greatly exceed the average light intensity along the lateral direction of the laser mirror. As the average optical intensity increases with increased pumping, these local areas will reach the threshold for optical damage much sooner than would be indicated by the average optical intensity alone, leading to premature failure when these lasers are operated to emit high output power.
The second effect is the large divergence of the output optical beam from a conventional broad area semiconductor laser. As indicated above, the existence of filamentation can be described as the excitation of selected perturbing waves. Thus, under these conditions, the optical output beam contains both the primary laser mode, which propagates in the optical cavity axis direction, and perturbing waves which propagate at an angle with respect to the optical cavity axis. As total laser power is increased by increasing the pumping of the broad area semiconductor laser, a greater proportion of the laser power appears in the perturbing waves and the output laser beam becomes increasingly divergent. This increased divergence may exceed the diffraction-limited divergence by a large amount, a actor of 10 or more. The combination of reduced reliability and increased output laser beam divergence makes conventional broad area lasers unsuitable for many applications.
There are some broad area semiconductor lasers which contain a grating within the waveguide layers. The grating is tuned so as to be in Bragg resonance at a certain wavelength within the gain bandwidth of the laser. Thus the grating is used as a filter tuned to select a single lasing wavelength or a single spatial mode. In other words, the grating is designed to increase the threshold gain for a number of spectral or spatial modes relative to the single mode for which the grating is in resonance.
U.S. Pat. No. 5,103,456 to Scifres et al., the entire disclosure and contents of which is hereby incorporated by reference, describes an integrated semiconductor master oscillator-power amplifier (MOPA) having a broad area gain region. For optical coupling between the oscillator and the amplifier, the patent describes the use of an angled grating reflector. The orientation angle and grating period are chosen to produce efficient Bragg diffraction to deflect the output oscillator laser beam in the direction of the amplifier optical axis, thereby minimizing the feedback from the amplifier into the oscillator.
U.S. Pat. No. 5,337,328 to Lang et al., the entire disclosure and contents of which is hereby incorporated by reference, describes a broad area semiconductor laser which includes a grating reflector oriented at an angle with respect to the optical resonant cavity mirrors. The grating period and orientation are chosen such that the wavelength and propagation direction of the light beam which satisfies the Bragg resonance condition over the total area inside the laser cavity will impinge at normal incidence to the optical cavity mirrors. The light beam completing a round trip through the optical resonant cavity takes a serpentine path due to Bragg reflections from the grating. Other light beams, which do not satisfy the Bragg resonance conditions because of a difference in wavelength or propagation direction, will incur additional optical losses and will have a correspondingly lower Q factor. The grating in this laser thus operates as a spectral and spatial filter. In so doing, Lang et al. attempt to create a semiconductor laser design that will suppress those laser beams which cause filamentation and thereby achieve single spatial and longitudinal mode operation.
It is an object of the present invention to provide broad area semiconductor lasers and amplifiers that can discriminate against filamentation in the lateral direction by introducing a lateral optical anisotropy in the active region of the laser.
It is further an object of this invention to provide a broad area semiconductor laser that is able to reduce the distortion of the planar wavefront of the output laser beam. This wavefront distortion may exist inside the laser cavity due to imperfect laser structure fabrication leading to inhomogeneities, temperature gradients during laser operation, current spreading, etc. In other words, it is an object of the present invention to incorporate a means for self-correction of the laser beam distortions that tend to become evident with increasing output power due to nonlinear wave interactions. The end goal is to obtain a high efficiency, high power, broad area semiconductor laser with diffraction limited divergence of the output laser beam.
In a first aspect, the present invention provides a coherent light-generating semiconductor structure comprising: at least one first active region including a first pumped portion for achieving optical gain; a first optical waveguide for confining light waves in the transverse direction and for propagating light waves along the optical axis of said first optical waveguide; first and second reflective means, the first and second reflective means being spaced apart from one another and defining a first resonant optical cavity therebetween, the first pumped portion of the at least one first active region being located at least partially within the first resonant optical cavity, and the first resonant optical cavity encompassing at least a portion of the first optical waveguide; the first active region, the first optical waveguide, and the first resonant optical cavity having properties necessary to establish and maintain a first principal optical mode; a first grating located in at least a portion of the first resonant optical cavity, the first grating having first grating teeth oriented at an angle relative to the first and second reflective means, such that substantially all excited light waves which are incident on the first and second reflective means are out of the Bragg resonance bandwidth of the first grating.
In a second aspect, the present invention provides a method for reducing filamentation in a high-power laser or amplifier comprising the steps of: generating a laser beam; and transmitting the laser beam through a resonant optical cavity defined by two reflective means spaced apart from one another and having a grating located therein, the grating having grating teeth oriented at an angle relative to the first and second reflective means such that substantially all excited light waves that become incident on the reflective means are out of the Bragg resonance bandwidth of the grating.